The present technology relates to methods for preparing shaped rubber articles from processable and dimensionally stable rubber compositions. In particular, the technology also relates to rubber articles cured by irradiation and methods for production thereof.
Cured elastomers or rubbers have a variety of physical properties useful for molded article applications. Among these properties are a high degree of flexibility, elasticity, resistance to compression set and sealability. Particular applications in which rubber compositions may be used include, for example, seals, gaskets, o-rings and hoses.
Conventional elastomer compositions in an uncured or unvulcanized state are generally processable and may be shaped, for example, by extrusion or molding. Curing of shaped elastomer compositions may further be carried out under elevated temperature and pressure conditions suitable for crosslink formation. Since elastomer compositions are thermoset or crosslinked during the curing process, the elastomer material is no longer processable after being fully cured.
Thermoplastic materials also provide a variety of desirable properties. In comparison to elastomers, thermoplastics exhibit excellent processability and environmental stability, e.g. chemical resistance and solvent resistance. Thermoplastic materials further exhibit excellent green strength characteristics. Green strength, or the resistance of a composition to sag, is necessary for maintaining the dimensional stability of shaped articles after the fabrication process.
In view of the beneficial properties inherent in both elastomer and thermoplastic materials, it has been desirable to admix suitable materials when forming rubber compositions. When elastomer and thermoplastic materials are combined, the materials can be present as co-continuous phases. In general, however, the materials are present as a continuous phase (also known as a major phase) and a dispersed phase (also known as a noncontinuous, discrete, or minor phase). When elastomer is present as a continuous phase, the composition may exhibit excellent elasticity and sealability.
Although the final rubber articles prepared by curing such compositions provide the desired blend of properties, the uncured compositions exhibit poor green strength. The poor green strength is believed to be due to the high elastomer content of such compositions. As a result, shaped but uncured elastomer compositions have a tendency to slowly deform under gravitational stress in a phenomenon known as creep. At room temperature, the creep phenomena may be called cold flow. Higher levels of thermoplastic, which do not flow or creep substantially at room temperature, are also not usable due to undesirable compression set and flexibility of final cured products.
There is a continuing need for a polymer composition combining the rubber properties of elastomer compositions with the processability and environmental stability of thermoplastic compositions. In particular, it is desirable to produce a processable elastomer composition having a green strength sufficient to maintain the dimensional stability of shaped uncured compositions prior to curing, and cured rubber articles having elastomeric properties and improved environmental stability.